Fiber Bragg Gratings in Carbon-Coated Optical Fibers and Techniques for Making Same

ABSTRACT

A technique is described for fabricating one or more optical devices in a carbon-coated optical fiber. A photosensitive optical fiber is provided having a hermetic carbon coating. Further provided is a laser having a beam output that is configured to inscribe one or more refractive index modulations into the optical fiber through the hermetic carbon layer while leaving the hermetic carbon layer intact. The laser is used to inscribe one or more optical devices into the optical fiber through the hermetic carbon layer.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a divisional of co-pending U.S. patentapplication Ser. No. 15/144,563, filed on May 2, 2016.

U.S. patent application Ser. No. 15/144,563 is a divisional of U.S.patent application Ser. No. 14/169,541, filed on Jan. 31, 2014, whichissued as U.S. Pat. No. 9,353,001 on May 31, 2016.

U.S. patent application Ser. No. 14/169,541claims the priority benefitof U.S. Provisional Patent Application Serial No. 61/784,347, filed onMar. 14, 2013, now expired.

All of the above applications are owned by the assignee of the presentapplication, and are incorporated herein by reference in their entirety.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates generally to the field of fiber optics,and in particular to fiber gratings and like optical devices for use inharsh environments.

Background Art

The demand for optical fibers and fiber-based devices has dramaticallyincreased over the past decade. However, in humid or hydrogen-richenvironments, fiber-based devices, such as sensors or other devicesemploying fiber Bragg gratings (FBGs), typically require shielding inorder to address mechanical robustness and reliability issues, as wellas issues relating to optical functionality. The reliability and opticalperformance of these devices are of primary importance to designers ofoptical components and systems.

There is thus an ongoing need in the art for FBGs and other fiber-baseddevices that are capable of reliable performance in harsh environments.

SUMMARY OF INVENTION

An aspect of the invention is directed to a method for fabricatinggratings or like optical devices in a carbon-coated optical fiber. Aphotosensitive optical fiber is provided having a hermetic carboncoating. Further provided is a laser having a beam output that isconfigured to inscribe one or more refractive index modulations into theoptical fiber through the hermetic carbon layer while leaving thehermetic carbon layer intact. The laser is used to inscribe one or moreoptical devices into the optical fiber through the hermetic carbonlayer.

Further aspects of the invention are directed to structures andtechniques for mass production of carbon-coated optical devices. In onepractice of the invention, optical devices are written into acarbon-coated optical fiber in a post-secondary-coating process. Inanother practice of the invention, optical devices are written into afiber on the draw tower during the optical fiber drawing process,subsequent to the application of a carbon coating onto the fiber.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 show diagrams, respectively, of a test optical fiber and agrating inscription setup used in an experimental demonstration of theinvention.

FIGS. 3-6 shows a series of reflection spectra illustrating the resultsof tests conducted using the fiber and the grating inscription setupshown in FIGS. 1 and 2.

FIG. 7A shows an isometric view of a carbon-coated optical fiberconfigured for use in a post-secondary-coating inscription technique, inaccordance with an aspect of the invention.

FIG. 7B shows a cross section of the carbon-coated optical fiber shownin FIG. 7A through the plane 7B-7B.

FIG. 8 shows a schematic diagram of an automated grating writing systemfor use with the fiber shown in FIGS. 7A and 7B.

FIG. 9 is a schematic diagram of an automated system according to anaspect of the invention for inscribing gratings into a carbon-coatedoptical fiber as part of the draw process.

FIG. 10 shows a flowchart of a general technique according to theinvention.

DETAILED DESCRIPTION

Aspects of the invention are directed to optical devices, such as fiberBragg gratings (FBGs) and the like, that are fabricated in an opticalfiber having a hermetic carbon coating. The structures and techniquesdescribed herein can be used, for example, to fabricate FBGs for use assensors in a harsh environment. In addition, the described structuresand techniques can be used to fabricate FBGs displaying improved fatigueresistance.

As used herein, the term “harsh environment” generally refers to anenvironment in which a hermetic carbon coating is useful in protectingan optical device. Such an environment includes, for example: ahigh-humidity environment, a hydrogen-rich environment; an environmentthat is both high-humidity and hydrogen-rich; a high-water-content oraqueous environment, such as liquid water; or the like. Generallyspeaking, hermeticity to water improves mechanical reliability in aharsh environment, and hermeticity to hydrogen improves opticalreliability.

As used herein, the terms “hermetic carbon coating” and “carbon coating”refer to a coating applied to a fiber surface so as to hermetically sealthe portion of the fiber contained within the carbon coating. The carboncoating serves a number of purposes, including, for example: providingthe glass fiber surface with a seal against water molecules, includingcondensed water, water vapor at a wide range of temperatures, and thelike; protecting against ingress of hydrogen or deuterium to ˜130° C. orhigher, depending on the coating structure; and extending the expectedlife of a given fiber in a given environment by retarding fatigue and byreducing stress corrosion failures.

In one carbon-coating technique, a carbon layer is applied onto theouter surface of an optical fiber in a draw tower as part of the drawingprocess. The draw tower is equipped with a carbon reactor, through whichoptical fiber travels after it has been drawn from a preform. Within thecarbon reactor, a chemical vapor deposition (CVD) technique is used todeposit a hermetic layer of amorphous carbon at a temperature of 1200°C. onto the outer surface of the optical fiber. A typical thickness forthe carbon coating is ˜0.02 μm to ˜0.08 μm.

The CVD technique is implemented by causing reactant gases to flowthrough the carbon reactor in the absence of oxygen. A pyrolyticreaction occurs at the glass surface of the drawn fiber, resulting inthermal deposition of the carbon onto the fiber surface. A minimumthickness of deposited carbon is required in order for the carboncoating to be hermetic. Typically, a polymer coating is then appliedover the carbon layer.

A carbon-coating technique is described in greater detail, for example,in Lindholm et al., “Low Speed Carbon Deposition Process for HermeticOptical Fibers,” International Wire and Cable Symposium (1999).

For a number of reasons, carbon coatings have not been used inconnection with FBGs and like optical devices. First, gratings may notbe inscribed into a fiber prior to the application of a carbon coatingbecause the high temperatures required to apply the carbon coating maydestroy the gratings. Second, previously, it has generally been believedthat gratings cannot be inscribed into a fiber subsequent to theapplication of a carbon coating, because of the carbon coating's lack oftransparency with respect to a UV laser beam.

The present invention is based on the insight that, if a fiber has asufficiently high degree of photosensitivity, and if the inscribinglaser is configured to operate within a suitable power range, it ispossible to apply a hermetic carbon coating onto the fiber's claddingand then subsequently inscribe gratings into the fiber through thecarbon coating without breaching the hermetic seal.

Generally speaking, the inscription of gratings into carbon-coatedoptical fibers can employ any of the configurations and techniques thatare used for inscribing gratings into optical fibers without carboncoatings. Thus, gratings can be inscribed into a carbon-coated opticalfiber using a holographic technique, a phase-mask technique, or thelike. Depending upon a particular situation, some adjustments ormodifications may be necessary.

As discussed below, the inscription of gratings into a carbon-coatedoptical fiber can be performed as part of the initial draw process or ina separate, post-secondary-coating process.

Photosensitivity

As noted above, in order for gratings to be written into a fiber througha carbon coating, the fiber must have a suitable degree ofphotosensitivity. The degree of photosensitivity required in a givensituation depends upon a number of factors, including (1) the strengthof the gratings to be inscribed and (2) the interaction between theinscribing laser beam and the carbon coating through which the gratingsare to be written.

According to an aspect of the invention, a suitable degree ofphotosensitivity in a given fiber is achieved through the use of one ormore dopants that are known to create photosensitivity. These dopantsinclude, for example, germanium, as well as fluorine, boron, or thelike. Depending upon a particular application, it may be desirable toemploy one or more co-dopants in one or more fiber regions in order toarrive at a fiber design having a desired degree of photosensitivity aswell as a particular refractive index profile or other desired property.

A suitable fiber for a given application of the present invention may beprovided in any of a number of different ways. For example, in certainsituations it may be possible to employ an already existing fiberdesign, if the fiber has a suitable degree of photosensitivity.Alternatively, an already existing fiber design may be modified bymaking suitable adjustments to its photosensitivity. In some situations,it may be necessary or desirable to custom design a suitable opticalfiber. The design of such an optical fiber will be understood adequatelyby a practitioner in the art to include, e.g.: core, trench, claddimensions and refractive index, as well as the possibility ofincorporating other structures, such as multiple cores,polarization-maintaining stress rods, a star-shaped or octagonalcladding, a rectangular core, one or more coatings of various types, andthe like.

According to a further aspect of the invention, hydrogen loading ordeuterium loading may be used to enhance the photosensitivity of thecarbon-coated fiber, and thus the reflectivity (i.e., strength) ofgratings written into the fiber. As mentioned above, a carbon coatingprotects against hydrogen ingression to ˜130° C. or higher, depending onthe coating structure. Thus, it is possible to load a carbon-coatedoptical fiber with hydrogen or deuterium, prior to inscription, byplacing the fiber in a pressurized chamber containing hydrogen ordeuterium at a suitable concentration and maintaining a fibertemperature above ˜130° C. for a suitable amount of time. The parametersfor such a conditioning schedule (e.g., time, temperature, and pressure)are dependent upon the required photosensitivity of the fiber, thehermeticity of the carbon coating, and the capability of the secondarycoating to survive this exposure.

Gratings are subsequently inscribed into the fiber in accordance withthe techniques described herein. According to a further aspect of theinvention, once the gratings have been inscribed into the fiber, theyare then annealed at a temperature of greater than ˜130° C. to allow theexcess loaded hydrogen or deuterium to escape from the fiber.

Experimental Confirmation

Aspects of the invention have been confirmed in a series of experiments,in which gratings were successfully inscribed into a carbon-coated testfiber. The successful outcome of these experiments has led to thedevelopment of a number of manufacturing techniques, discussed below,that can be used for mass production of carbon-coated gratings.

FIGS. 1 and 2 show diagrams, not drawn to scale, of the test fiber 10and an inscription station 20. The test fiber 10 was a single-core fibercomprising a core region 12, a cladding region of 80 μm in diametersurrounding the core 14, a hermetic carbon layer 16, and a protectivepolyimide coating 18 surrounding the carbon layer 16. The fiber coreregion 12 was doped with germanium at a concentration resulting in arelatively high degree of photosensitivity and a numerical aperture of0.21. The carbon coating 14 was applied during the draw process and hada thickness of approximately 0.029 μm. The polyimide coating 18 was alsoapplied during the draw process, subsequent to the application of thecarbon layer, and had a thickness of approximately 15 μm.

It is noted that, depending upon a particular application, a carboncoating having a thickness as high as 0.08 μm, or thicker, may beconsidered.

The fiber 10 was loaded into the inscription station 20, which includedmeans (not shown) for holding and positioning the fiber relative to theoutput of laser 22 and beam delivery optics 24. Prior to inscription,the polyimide coating 18 was stripped from the fiber, leaving the carboncoating 16 intact.

In the FIG. 2 inscription station 20, the laser 22 is implemented usingan excimer UV laser manufactured by TuiLaser, which provided a laserbeam having a wavelength of 248 nm. A phase mask 26 was used to generatea periodic interference pattern from the UV laser beam, which was usedto inscribe a grating 28 into fiber core 12, or fiber core 12 and partof cladding 14, depending on the fiber design. It will be appreciatedthat the present invention can be practiced with other suitable types oflasers, depending upon a given application.

It is noted that there is a relatively wide range of wavelengths atwhich a carbon coating is sufficiently transparent to allow gratings tobe written therethrough. These wavelengths include 193 nm, 244 nm, and248 nm. Longer wavelengths may be employed, such as when using a“cold-writing” technique, a femtosecond laser, or other approaches. Inthe tests, a number of gratings were inscribed into the carbon-coatedtest fiber using different inscription parameters. The performance ofthe inscribed gratings was measured using a setup having a broadbandsource, a 3-port circulator and an optical spectrum analyzer (OSA).FIGS. 3-5 show a series of reflection spectra illustrating the testresults. Among other things, the reflection spectra confirm that it ispossible to inscribe gratings into a carbon-coated optical fiber. Inaddition, the reflection spectra confirm that, generally speaking,stronger FBGs can be written by longer exposure to the inscribing laserlight and/or increasing the concentration of germanium (or othersuitable dopant or combination of dopants) in the fiber into which thegrating is written.

FIG. 3 shows a reflection spectrum 30 generated by an 8 mm gratinginscribed into a length of the test fiber, as described above. TheTuiLaser excimer laser was used to provide a 40 Hz pulsed laser beamhaving a wavelength of 248 nm. The laser energy density in theinscription region was ˜25 mJ/cm². The exposure time was 60 seconds,after which time the grating did not change significantly. The laserenergy density was chosen because at ˜25 mJ/cm² the carbon coatingshowed no visual damage when inspected under a microscope. Experimentshave shown that a power density level of 80 mJ/cm² will burn or damagethe carbon coating. The exact upper limits for the inscriptionparameters have not yet been determined, but are expected to be between25 mJ/cm² and 80 mJ/cm² for the 248 nm TuiLaser excimer laser. For otherwavelengths or types of lasers, the laser energy density ranges may bedifferent.

As shown in FIG. 3, the 8 mm grating reflection spectrum reaches a peakof ˜−11 dB, which corresponds to a peak reflectivity of ˜8%.

FIG. 4 shows the reflection spectrum 40 of an FBG written into acarbon-coated optical fiber by a single laser pulse (i.e., a “one-pulse”inscription process). The setup used was exactly the same as the oneused to inscribe the FBG in FIG. 3. The one-pulse FBG achieved a peakreflectivity of ˜0.29% (˜−25.5 dB). This result is significant becauseit implies that the carbon-coated fiber gratings could be fabricated byusing a draw tower grating writing technique.

The grating illustrated in FIG. 3 is the stronger of the two, and issuitable for use, for example, in a wavelength division multiplexing(WDM) based interrogator. The weaker one-pulse grating illustrated inFIG. 4 is suitable for use, for example, in a weak grating interrogatorbased on time division multiplexing/wavelength division multiplexing(TDM/WDM) techniques. In a TDM/WDM system, the reflectivity ofindividual FBGs can be as low as −35 dB (i.e., a reflectivity of˜0.03%).

FIG. 5 shows the reflection spectra 50 of a series of four 8 mm gratingswritten into the carbon-coated test fiber using the same testing setup,but with a different number of laser pulses (one pulse 51, two pulses52, three pulses 53, and five pulses 54).

A further experiment was conducted to determine whether the inscriptionof gratings in the test fiber resulted in damage to the carbon coatingnot visually observed under a microscope.

Six FBGs similar to the one shown in FIG. 3 were inscribed into the testfiber. Their wavelengths were measured at room temperature afterannealing for 64 hours at 85° C. The six FBGs were then placed into adeuterium exposure chamber having a pressure of 4,400 psi pressure and atemperature of 50° C. After 20 days, the FBGs were removed from thechamber and their wavelengths were measured again at room temperature.Deuterium gas was chosen because the setup for creating the abovedeuterium environment was readily available. It is expected that thesame results should be obtained by replacing deuterium gas with hydrogengas.

Compared to the wavelengths before the loading, the wavelength shiftsfrom these FBGs ranged from −12 pm to 0 pm with an average wavelengthshift of −7.2 pm. Given a variation in room temperature of ˜0.5° C. andthe relatively small wavelength shifts, it can be concluded thatdeuterium did not enter into the fiber core with those loadingconditions. It can also be concluded that the grating writing did notdamage the carbon coating.

FIG. 6 shows a reflection spectrum 60 for a grating with a length of 10mm that was written into another carbon-coated fiber, in which thecarbon coating thickness was ˜0.056 μm, the cladding diameter was 125μm, and the numerical aperture was 0.16. The laser conditions were thesame as those in the tests of the gratings inscribed into carbon-coatedoptical fibers, in which the carbon coating had a thickness of ˜0.029μm. In reflection spectrum 60, the 0.6% peak reflection 61 is referencedto the cleaved fiber end reflection of 4%. Compared to the resultsobtained from the 0.029 μm carbon coated fiber, this lower reflectivityis believed to be mainly due to both the less photosensitive fiber andthe thicker carbon coating. To verify this, gratings were also writtenin a fiber with the same glass structure as that of the 0.056 μm thickcarbon coated fiber, but a 0.024 μm thick carbon coating. The resultsshowed that 1.7% peak reflectivity was obtained for a 10 mm grating. Itis believed that with higher germanium doping in the fiber core andextra photosensitization of the fiber, much higher grating peakreflectivity can be obtained even in the thick carbon coated fiber.

Mass Production of Carbon-Coated Optical Fiber Gratings

The above encouraging results have given rise to a number of possiblescenarios for mass-producing FBGs in carbon-coated optical fibers. Inone approach, FBGs are inscribed into a carbon-coated optical fiber in apost-secondary-coating process. In a second approach, FBGs are inscribedinto a fiber as part of the draw process, after the application of acarbon coating. Each of these approaches is described in turn.

Post-Secondary-Coating Inscription

As used herein, the term “post-secondary-coating inscription” refers toan inscription process that is performed subsequent to the applicationof one or more secondary coatings over a carbon coating layer. A“post-secondary-coating inscription” technique can be performed in adraw tower as part of an optical fiber drawing process, or can beperformed in an inscription station after a fiber has been removed froma draw tower.

FIG. 7A shows an isometric view of a carbon-coated optical fiber 70 thatis configured for use in a post-secondary-coating inscription technique,in accordance with an aspect of the invention. FIG. 7B shows a crosssection of the carbon-coated optical fiber shown in FIG. 7A through theplane 7B-7B.

Fiber 70 comprises a core region 72, a cladding region 74, and ahermetic carbon layer 76. Surrounding the hermetic carbon layer is aprotective “write-through” coating 78 that is substantially transparentto ultraviolet light. Such a coating is described in U.S. Pat. No.5,620,495, which is owned by the assignee of the present application,which is incorporated herein by reference in its entirety. As disclosedtherein, the outer coating comprises a polymer with low UV absorption.Such a coating may comprise, for example, a polymer selected from thegroup consisting of acrylates, aliphatic polyacrylates,silesesquioxanes, alkyl-substituted silicones and vinyl ethers. As usedherein, the terms “acrylates” and “aliphatic polyacrylates” includemethacrylate, poly(meth)acrylate, urethane acrylate, and the like.

FIG. 8 shows a schematic diagram of an automated FBG writing system 80for use with fiber 70. System 80 comprises a programmable controllerunit 81, an automated fiber winding system 82, an automated fiberclamping system 83, and a UV laser 84. The beam delivery optics 85 canbe based on phase mask or holographic techniques. Fiber 70 is loadedinto the automated fiber winding system 82, and gratings are writteninto the fiber 70 in a series of inscription cycles. In each cycle, theclamping system 83 clamps a portion of fiber 70 in position relative tothe output of UV laser 84, as shaped by the beam delivery optics 85.

An FBG array fabricated using fiber 70 and grating inscription system 80maintains pristine fiber strength. Further, as discussed above, sincethe fiber is carbon-coated, it can be configured for use in humid,hydrogen-rich, or other harsh environments, such as those describedabove. The “post-secondary-coating” approach allows for greater degreesof manipulation to achieve strict optical spectrum requirements notnormally achievable with draw-tower grating writing.

Pre-Secondary-Coating Inscription

As used herein, the term “pre-secondary-coating inscription” refers toan inscription process that is performed subsequent to the applicationof a carbon coating layer, but prior to the application of a secondarycoating over the carbon coating layer.

In an exemplary practice of a pre-secondary-coating inscriptiontechnique according to an aspect of the invention, gratings areautomatically inscribed into a carbon-coated optical fiber in a drawtower as part of the draw process. FIG. 9 is a schematic diagram of anautomated system 90 according to this aspect of the invention.

System 90 comprises a draw tower 91 having a furnace 92 at its top thatis configured to receive a fiber preform from which a fiber 93 is drawn.The drawn fiber descends through the following stages: a carbon coatingapplicator 94, in which a hermetic carbon coating is applied to thefiber 93; a grating inscription stage 95, in which gratings areinscribed into the fiber; a secondary coating die 96, in which an outercoating is applied over the carbon coating; and a fiber winding system97 for collecting the finished fiber and winding it onto a bulk reel.

Grating inscription stage 95 comprises a UV laser 951 and laser beamdelivery optics 952 that are configured to deliver an inscription laserbeam 953 to a mask 954 and its mounting fixtures. Of course, othergrating inscription techniques, such as a holographic technique, canalso be implemented here. The amount of time required to inscribe eachgrating is sufficiently short, such that it is possible to inscribegratings into fiber 93 without having to stop the fiber as it descendsthrough the draw tower 91.

Because the outer coating is applied subsequent to the inscription ofgratings into fiber 93, it is not necessary for the outer coating to bea write-through coating. Thus, the described approach providesflexibility with respect to the outer coatings that can be applied bythe secondary coating die 96. Any of a number of different outercoatings may be used, including, for example, acrylate, polyimide, vinylether, silicone, silsesquioxane, epoxy, and even metal.

General Technique

FIG. 10 is a flowchart illustrating a general technique 100 according tothe above-described aspects of the invention.

Technique 100 comprises the following steps:

101: Provide a photosensitive optical fiber having a cladding onto whichhas been applied a hermetic carbon coating.

102: Provide a laser having a beam output that is configured to inscribeone or more refractive index modulations into the optical fiber throughthe hermetic carbon layer while leaving the hermetic carbon layerintact.

103: Use the laser to inscribe one or more optical devices into theoptical fiber through the hermetic carbon layer.

Conclusion

Fiber Bragg gratings fabricated in carbon-coated fibers in accordancewith the present invention have a number of advantages, including: (1)resisting hydrogen ingression below a certain temperature; (2)increasing long-term reliability when the fiber is under stress and in ahumid environment.

In both post-secondary-coating and pre-secondary-coating inscription ofFBGs in carbon-coated optical fibers, another advantage is that theFBG-inscribed carbon-coated optical fiber will have almost the samebreaking strength as that of the pristine carbon-coated optical fiber(˜550 kpsi), i.e., prior to inscription of the FBG therein.

While the foregoing description includes details that will enable thoseskilled in the art to practice the invention, it should be recognizedthat the description is illustrative in nature and that manymodifications and variations thereof will be apparent to those skilledin the art having the benefit of these teachings. It is accordinglyintended that the invention herein be defined solely by the claimsappended hereto and that the claims be interpreted as broadly aspermitted by the prior art.

What is claimed is:
 1. A method for fabricating one or more opticaldevices in a carbon-coated optical fiber, comprising: providing aphotosensitive optical fiber having a hermetic carbon coating and asecondary outer coating over the hermetic carbon coating, wherein eachof the hermetic carbon coating and the secondary outer coating has arespective absorption of light at a given wavelength that allows a laseroperating at the given wavelength to inscribe one or more opticaldevices into the fiber through the secondary outer coating and thehermetic carbon coating, while leaving them intact; providing a laserhaving a beam output that is configured to inscribe one or morerefractive index modulations into the optical fiber through the hermeticcarbon layer and the secondary outer coating while leaving the hermeticcarbon layer and the secondary outer coating intact; and using the laserto inscribe one or more optical devices into the optical fiber throughthe hermetic carbon layer and the secondary outer coating.
 2. The methodof claim 1, wherein the one or more optical devices comprise one or morefiber gratings.
 3. The method of claim 2, wherein a phase mask techniqueis used to inscribe the one or more gratings into the fiber.
 4. Themethod of claim 2, wherein a holographic technique is used to inscribethe one or more gratings into the fiber.
 5. The method of claim 1,wherein the one or more optical devices are written into the fiberduring draw, subsequent to the application of the carbon coating andsecondary outer coating onto the fiber.
 6. The method of claim 1,wherein the carbon coating and secondary outer coating are applied tothe fiber during draw, and wherein the one or more optical devices areinscribed into the fiber during a post-draw process.
 7. The method ofclaim 1, wherein the secondary outer coating comprises a polymer.
 8. Themethod of claim 1, wherein the laser is an ultraviolet laser, andwherein each of the hermetic carbon coating and the secondary outercoating has a respective ultraviolet absorption that is sufficiently lowso as to allow the ultraviolet laser to inscribe one or more opticaldevices into the fiber through the secondary outer coating and thehermetic carbon coating.
 9. The method of claim 1, wherein the laser isa femtosecond laser, and wherein the secondary outer coating has aninfrared absorption that is sufficiently low so as to allow thefemtosecond laser to inscribe one or more optical devices into the fiberthrough the secondary outer coating and the carbon coating.
 10. Themethod of claim 1, wherein the carbon layer has a thickness ranging from0.02 to 0.08 μm.
 11. A method for fabricating a photosensitive,carbon-coated optical fiber, comprising: doping a fiber to have a highdegree of photosensitivity; applying a hermetic carbon coating to thefiber cladding; and applying a secondary outer coating over the hermeticcarbon coating, wherein the secondary outer coating has an absorption ata given wavelength of light that is sufficiently low so as to allow theone or more optical devices to be inscribed into the fiber, in apost-secondary-coating process, through the secondary outer coating andthe carbon coating, using a laser operating at the given wavelength. 12.The method of claim 11, wherein the fiber core or fiber core and part offiber cladding are doped with one or more rare-earth dopants.
 13. Themethod of claim 12, wherein the one or more photosensitive dopantscomprises germanium.
 14. The method of claim 13, wherein hydrogen ordeuterium loading is used to increase the photosensitivity of the fiber.15. The method of claim 14, wherein hydrogen or deuterium is loaded intothe fiber subsequent to the application of the carbon coating by heatingthe fiber to a temperature at which the hydrogen or deuterium to beloaded passes through the carbon coating.
 16. A method for fabricating aphotosensitive, carbon-coated optical fiber, comprising: doping a fiberto have a high degree of photosensitivity; applying a hermetic carboncoating to the fiber cladding, wherein the carbon layer is hermetic toboth water and hydrogen, and wherein the carbon layer has a thicknessthat is sufficiently transparent to light at a given wavelength so as toallow an optical device to be written into the fiber through the carbonlayer; and using a laser operating at the given wavelength to write anoptical device into the fiber.
 17. A method for fabricating one or moreoptical devices in a carbon-coated optical fiber, comprising: providinga photosensitive optical fiber having a hermetic carbon coating and asecondary outer coating over the hermetic carbon coating; providing alaser having a beam output that is configured to inscribe one or morerefractive index modulations into the optical fiber through the hermeticcarbon layer while leaving the hermetic carbon layer intact; in aselected region of the fiber, stripping off the secondary outer coating,while leaving the carbon coating intact; and using the laser to inscribeone or more optical devices into the optical fiber through the hermeticcarbon layer.
 18. The method of claim 17, further including: after usingthe laser to inscribe one or more optical devices into the opticalfiber, applying a protective coating over the selected region of thefiber.
 19. A system for mass producing carbon-coated optical fibergratings, comprising: a draw tower; a furnace assembly at the top of thedraw tower for receiving an optical fiber preform, and for heating thepreform so as to allow an optical fiber to be drawn therefrom; a carbonreactor assembly located in the draw tower below the furnace assemblyfor receiving the drawn fiber and for applying thereto a hermetic carbonlayer; a grating inscription assembly located in the draw tower belowthe carbon reactor assembly for receiving the carbon-coated opticalfiber and for using a laser to inscribe gratings into the carbon-coatedoptical fiber through the hermetic carbon coating, while leaving thefiber's carbon coating intact; and an outer coating applicator assemblyfor applying an outer coating over the carbon-coated optical fiber andthe gratings inscribed therein.
 20. A system for mass producingcarbon-coated optical fiber gratings, comprising: a draw tower; afurnace assembly at the top of the draw tower for receiving an opticalfiber preform, and for heating the preform so as to allow an opticalfiber to be drawn therefrom; a carbon reactor assembly located in thedraw tower below the furnace assembly for receiving the drawn fiber andfor applying thereto a hermetic carbon layer; an outer coatingapplicator assembly for applying a secondary outer coating over thecarbon-coated optical fiber, wherein the secondary outer coating has anabsorption at a given wavelength that is sufficiently low so as to allowone or more optical devices to be written into the optical fiber throughthe carbon layer and secondary outer coating; and a grating inscriptionassembly located in the draw tower below the outer coating applicationassembly for receiving the optical fiber and for using a laser toinscribe gratings into the optical fiber through the carbon coating andsecond outer coating, while leaving the fiber's carbon coating intact.